The proposed work is directed towards a further understanding of the molecular mechanisms regulating gene expression, with particular emphasis on factors determining the specificity of these processes. We are analyzing the complex interactions of phage and bacterial proteins which regulate the choice between lytic and lysogenic growth by bacterial virus lambda in its host E. coli. In particular we will continue our studies concerning analysis of four phage regulatory genes--cro, cII, cIII, and Q- -whose gene products control synthesis of other lambda proteins, and a bacterial gene hfl which greatly influences the decision between lytic and lysogenic growth. Our isolation of lambda mutants which do not require cIII regulatory protein for efficient lysogenization strengthens the view that cIII indirectly stimulates repressor synthesis. Studies of these mutants indicate that the normal role of cIII is to increase activity of cII protein. Work of others has led to the suggestion that a bacterial function, coded by the hfl gene, may be an inhibitor of repressor synthesis. We shall test the hypothesis that cIII and Hfl proteins regulate activity of cII protein by direct assays of cII protein synthesis and stability. Our studies on the role of the cro gene have shown that Cro is essential for lambda growth for two reasons, to turn off premature repressor synthesis and to turn down synthesis of early proteins whose overproduction leads to defective phage growth. We have identified one of the inhibitors and shall utilize lambda mutants lacking the inhibitor to identify the other inhibitors. Because lambda Q protein is necessary for transcription of lambda genes and because host RNA polymerase is used for lambda transcription, we seek to determine whether Q and RNA polymerase interact physically. Two methods will be used to determine if this occurs: identification of bacterial mutants which do not need Q protein to recognize lambda control sites, and identification of bacterial mutants which are incapable of utilizing Q protein.